1. Field of the Invention
The present invention relates to a steel sheet for container, which is usable as a raw material for manufacturing cans, and which is in particular excellent in the processability (or workability) in a drawing and ironing procedure, weldability, corrosion resistance, adherence thereof to a coating material, and adherence thereof to a film. The present invention also relates to a process for producing such a steel sheet for container.
2. Related Background Art
Containers made of metals to be used for containing therein beverages or foods can be classified into two-piece cans and three-piece cans. In the manufacture of the two-piece cans represented by DI cans, after the drawing and ironing process for a raw material for the can, a coating material is applied to the inner surface side of the resultant can, and a coating material and printing are applied to the outer surface side of the can. In the manufacture of the three-piece cans, a coating material is applied to the surface of corresponding to the inner surface of the can, printing is applied to the surface corresponding to the outer surface side of the can, and thereafter the barrel portion of the can is subjected to welding.
In either case of the above types of cans, a coating step to be conducted before or after the can manufacturing step is an indispensable step. In the coating step, a solvent-based or water-based coating material is used, and thereafter, a baking step is performed. In this coating step, a waste matter (such as waste solvent) attributable to the coating material is discharged as industrial wastes, and the exhaust gas (mainly comprising carbon dioxide gas) is released into the atmosphere. In recent years, various attempts or approaches to reduce such industrial wastes and exhaust gases are being made. Among these, a technique of laminating a film has been attracting attention as an alternative to the above coating technique, and such a laminating technique has rapidly been spread.
With respect to the two-piece cans, a large number of can manufacturing processes including a step of laminating a film so as to manufacture a can, and a large number of inventions relating to such processes have heretofore been provided and reported. Examples the invention may include: “Process for manufacturing draw-ironed can (Patent Document 1)”, “Draw-ironed can (Patent Document 2)”, “Process for manufacturing a thin-walled deep-drawn can (Patent Document 3)” and “Coated steel sheet for draw-ironed can (Patent Document 4)”.
Further, with respect to the three-piece cans, examples of the such an invention may include “Film-laminated steel band (or strip) for three-piece can and process for producing the same (Patent Document 5)”, “Three-piece can having multilayer organic film on outer surface of can (Patent Document 6)”, “Steel sheet for three-piece can having striped multilayer organic film (Patent Document 7)” and “Process for producing striped laminated steel sheet for three-piece can (Patent Document 8)”.
On the other hand, with respect to the steel sheet to be used as an underlying layer of the laminate film, chromate films which have been subjected to an electrolytic chromate treatment have been used in many cases. The chromate film has a two-layer structure, where a hydrated Cr oxide layer is present on a metal Cr layer. Accordingly, the laminate film (in the case of a film with an adhesive, the adhesive layer) may ensure the adherence to the steel sheet through the medium of the hydrated Cr oxide layer of the chromate film. The details of the mechanism of developing such an adherence have not clearly been reported yet, but it has been supposed that the hydrogen bonding between the hydroxyl group of the hydrated Cr oxide and the functional group such as carbonyl group or ester group of the laminate film is attributable to the above adherence.